Allow platform specific external scripts to override options

With the current init script it is not possible to have different configurations for client and server as the run options are fixed before the platform specific scripts get to run (/etc/sysconfig/sensu-$prof). We use different a rabbitmq FQDN for client and server so we need to override config directory for server/client components. This patch moves the code which sets command line options to run after the platform specific scripts have run, allowing this specific customisation over and above that applied to all components by /etc/default/sensu.